Up to date, a systemic treatment of cancer is based mainly on the use of chemotherapy. However, in the majority of cases, chemotherapy is not a radical treatment. In initially identified tumors there already exist cells that are resistant to toxic drug action, due to their biochemical properties. Furthermore, the proportion of such cells is progressively increased throughout the treatment period because they receive selective growth advantages over the cytotoxic drug-susceptible cells. It should also be noted that cytotoxic action of antineoplastic drugs is not selective: the drugs affect not only tumor, but also normal cells. Hence, there remains a need for the drugs with selective cytotoxic activity.
Tumor cells are distinguished from normal ones by quantitative and qualitative expression on their surfaces of potentially immunogenic structures (antigens). It is generally accepted that the immune responses induced by these structures can cause destruction of tumor cells, and that reactivity of the immune system can define the outcome of disease. All of the tumor-associated antigens (TAAs) can be divided into two groups: the first one involves the differentiation antigens that can be expressed not only in tumor, but also in normal cells, whereas the second one comprises of the products of mutated or viral genes, which can be expressed exclusively in malignant cells. The vast majority of TAAs belongs to the first group. Some of TAAs in this group, for example cancer/testis antigens (CTAs) can be expressed in a variety of tumors, due to commonality in the intracellular mechanisms involved in malignization of various types of cells. Other TAAs (for example oncofetal antigens) are defined by a type of a tumor and are mostly represented by tissue-specific differentiation antigens (reviewed by Strioga et al., 2014). In an adult organism CTAs are normally expressed only in immune privileged organs including as testis and placenta, and can be aberrantly expressed in cancer cells. For example, in the adult body the products of gene families MAGE, BAGE, GAGE and some others are mainly expressed in the testis rather than in other tissues and organs. On the other hand, many types of tumors may share expression of these CTAs (reviewed by Fratta et. al. 2011). CTAs possess a high immunogenic potential since they are “unknown” to the immune system and hence are not tolerated. Oncofetal antigens may be normally expressed at very low levels in normal tissues (for example alpha-fetoprotein in the liver) and can be overexpressed in some cancers or during various non-malignant pathologies. The overexpressed oncofetal antigens are less immunogenic than CTAs (Strioga et. al., 2014)
It should be noted that immunizations with one or several tumor-associated, antigenic peptides frequently fail to control overall tumor development, creating favorable conditions for growth of the tumor cell clones lacking vaccinal determinants. Moreover, due to a high lability of cancer genome, there is an antigenic diversity even in tumor cells of the same origin (reviewed by Khong H T, Restifo N P., 2002).
Since whole tumor cells express a variety of TAAs and are able to elicit a broad spectrum of immune responses, they could be more applicable to constructing cancer vaccines, compared to a single or just a few antigenic peptides. Moreover, antigenic cellular particles are usually much more immunogenic compared to soluble antigenic peptides, due to their ability to be wholly phagocytized by professional antigen-presenting cells capable of presenting identical, cell-derived peptides in association with major histocompatibility complex (MHC) molecules at a density sufficient to trigger T-cell responses.
Various types of tumor cell-based vaccines have been developed. Autological (made from the tumor cells of the same individual) and allogeneic (made from the tumor cells of a different individual of the same species) whole-cell vaccines, as well as vaccines on the basis of dendritic cells have been used for induction of specific antitumor immune responses (de Gruijl et. al., 2008; Itoh et. al., 2009). However, immunizations with unmodified homologous (autological or allogeneic) tumor cells have demonstrated only limited therapeutic success in cancer patients. There are two major reasons for the low immunogenicity of homologous cell vaccines. Firstly, as mentioned above, most of TAAs represent self-antigens, which are not inherently immunogenic. Secondly, antigen-presenting cells do not recognize the homologous tumor cells as potentially pathogenic targets that should be internalized and their antigens processed (Khong H T, Restifo N P., 2002). Accordingly, overcoming immune tolerance towards TAAs is a key task of cancer immunotherapy.
Certain approaches have been made to boost immunogenicity of autologic or allogeneic cancer vaccines, based on genetic modifications of vaccinal cells, rendering them superficially expressing costimulatory molecules and/or secreting immunostimulatory cytokines (de Gruijl et. al. 2008; Andersen et. al., 2008). However, all these kinds of modification are difficult to achieve in clinical practice as modification of tumor cells is technically complicated and time-consuming (reviewed by Parmiani et. al., 2011).
The use of xenogeneic TAAs has been suggested to overcome the immune tolerance to homological self TAAs. Indeed, many genes are highly evolutionarily conserved with various degrees of similarities among different species. Nevertheless, the small interspecific structural differences may confer increased immunogenicity to xenoproteins and provide a marked immune cross-reactivity directed against their homologous counterparts. In fact, xenoantigens may potentially represent an “altered self”, with sufficient differences from self-antigens to render them immunogenic, but with sufficient similarities to allow reactive T cells to maintain recognition of self (reviewed by Seledtsov et. al., 2011; Strioga et. al., 2014). There is evidence that xenoepitopes can bind host MHC molecules more strongly than epitopes derived from native homologous proteins, resulting in the formation of more sustained xenogeneic peptide/MHC complexes. Ultimately this leads to more potent xenoantigen-induced T cell responses, cross-reactive with self-protein-derived TAAs (Overwijk et. al., 1998).
The majority of studies concerning xenogenic vaccines have been carried out on animals with melanoma, the tumor that expresses a whole number of potentially immunogenic antigens. There is compelling evidence that xenogenic melanoma-associated antigens are much more effective in inducing antitumor immune responses in mice than are their murine analogs. For example, multiple immunizations of mice with human glycoproteins gp75 and gp-100 have been reported to be effective in preventing the growth of the syngeneic melanoma cells expressing the appropriate mouse analogs (Overwijk et. al., 1998; Weber et. al., 1998). Immunogenic and antitumor effects of xenovacination have been also reported in experimental models of hepatocellular carcinoma, glioma, neuroblastoma, colon cancer, and lung carcinoma. Theraupeutic vaccinations were found to be capable of generating tumor-specific CD4+ and CD8+T lymphocytes, as well as antitumor antibodies (reviewed by Strioga et. al., 2014).
The data indicating the therapeutic potential of antitumor xenovaccination in humans are also accumulating. For example, a vaccine consisting of murine melanoma and carcinoma cells, as well as porcine testis cells has been reported to be immunologically and clinically effective in certain patients with melanoma and colorectal cancer (patent RU2192884C2). The published results suggest that xenogenic vaccines are safe to use, able to induce measurable cellular and humoral immune responses in patients, and may serve as effective means for treating melanoma, renal cancer, tumors of digestive system, lung cancer and prostate cancer (Seledtsov et. al., 2011).
It is important to note that all humans possess natural (preexisting) antibodies (Abs) that provide an acute rejection of any non-primate cells and function as a major barrier for transplantation of animal organs in humans. A significant part of these Abs represents IgG specific to the alpha-gal epitope that is abundantly expressed on glycoproteins and glycolipids of nonprimate mammals and NewWorld monkeys (Galili U., 1993) By opsonization of xenogenic cells, the natural Abs promote internalization of antigenic material in antigen-presenting cells APC via a Fcg-receptor-mediated mechanism, and enhance greatly the immunogenic cross-presentation of antigenic peptides to antigen-specific CD4+ and CD8+T lymphocytes (Galili U., 1993; Platzer et. al., 2014). This proposition is consistent with the published results showing that rejection of alpha-Gal positive tumor cells can efficiently boost the immune response to other tumor-associated antigens present in alpha-Gal negative tumor cells (Rossi et. al., 2005).
The xenogenic cell-based vaccines are typically presented in the form of whole tumor cells or their lysates (Seledtsov et. al., 2011). However, using tumor cell cultures is rather expensive and they are not always reproducible sources of TAAs. It is also important to know that although the tumor cells used in the xenogenic vaccines are not vital, such vaccines may still contain components that could be involved in tumorogenesis.
As already mentioned, the vast majority of TAAs belongs to the differentiation antigens that are highly expressed in normal cells involved in organ morphogenesis. This raises the possibility of obtaining xenogenic vaccinal antigens from normal tissues where they are highly expressed. For example, cancer/testis antigens (CTAs) pertaining to so-called common TAAs can be readily obtained from normal testicular tissues and could be subsequently used as universal antitumor immunogens. According to published data (reviewed Lim et. al. 2012), immune responses directed against CTAs may cause tumor destruction, while not damaging normal tissues and organs. Normal fetal-derived tissues can be suitable sources for vaccinal oncofetal antigens. Placenta is also well known to express a whole range of differentiation antigens, including those shared by different tumors including melanoma (Zhong et al, 2006).
Despite the above said, there remains a need for a cancer vaccine, which would be clinically effective, easily prepared, reproducible and inexpensive. It is also highly desirable that a cancer vaccine-based technology could be applicable for treatment or prevention of more than one type of cancer or its specificity could be easily changed. Xenogenic normal tissue-derived vaccines may conceivably meet all these requirements.